    [Patent Document 1] JP-A-2000-2284    [Patent Document 1] JP-A-10-47310
Hydraulic pressure springs such as shock absorbers, dampers, and the like using a compressible liquid are used in the fields of, for example, rolling stocks, fender devices, production machines, and the like in portions which are required to dampen and absorb large impact energy.
In a hydraulic pressure spring, its performance varies substantially according to the internal pressure imparted to the compressible liquid in its initial state.
With an elastic fluid compression type shock absorber as a hydraulic pressure spring such as the one shown in Patent Document 1, when a piston rod forcibly enters the interior of the cylinder owing to a shock, the volume for accommodating an elastic fluid in the cylinder becomes small as compared with the initial period, so that the elastic fluid sealed in the cylinder is compressed and its pressure increases. This increased pressure serves as a reaction force, and the spring action (restoring action) is generated to push back the impact object and absorb the energy in the process in which the elastic fluid is compressed.